1. Technical Field
The present invention relates to a rear wheel drive apparatus for a hybrid electric vehicle, and more particularly, to a rear wheel drive apparatus for a four wheel drive (4WD) hybrid electric vehicle which includes a motor is generator for driving rear wheels, a reduction gear set for reducing an output speed of the motor generator and transmitting the decelerated output, and a differential gear set for appropriately distributing torque transmitted through the reduction gear set to wheel shafts of rear wheels so as to have different rotation speeds.
2. Background Art
Generally, a hybrid electric vehicle (HEV) is a vehicle which efficiently combines two or more types of power sources to drive itself. A typical hybrid electric vehicle obtains driving force through an engine using fuel and an electric motor driven by electric power of a battery.
Researches on a hybrid electric vehicle have been actively undergone in accordance with the demand of the times for improving a fuel consumption ratio and development of environment-friendly products.
A hybrid electric vehicle has an electric vehicle (EV) mode which is a pure electric vehicle mode in which a vehicle is driven by torque of a motor selectively according to a state of charge (SOC) of a battery and a driving state, an engine mode in which a vehicle is driven by torque of an engine, a HEV mode which is an auxiliary mode which uses torque of an engine as a main power and torque of a motor driven by a battery power as an auxiliary power, and a regenerative braking (RB) mode in which braking and inertia energy are collected through generation of electric power in a motor to be charged into a battery during driving using braking or inertia of a vehicle.
A hybrid electric vehicle has a structure which appropriately combines two power sources, i.e., engine and motor and has an improved fuel consumption ratio due to power support of a motor driven by a battery power when a vehicle starts or accelerates.
In a 4WD hybrid electric vehicle, a reduction gear set reduces an output speed transmitted from an output shaft of a rear wheel driving motor and transmits the reduced output speed to a differential gear set. Before transmitting the electric power to a wheel shaft from the output shaft of the driving motor, the reduction gear set reduces the rotation speed from the output shaft of the driving motor to increase torque, thereby finally achieving a deceleration.
The differential gear set appropriately distributes torque transmitted through the reduction gear set to respective wheel shafts of rear wheels to have different rotation speeds. The differential gear set automatically makes a rotation difference when a rotation difference between left and right wheels is required in case where a vehicle turns or moves on an uneven road surface, leading to smooth driving.
FIGS. 1 to 3 are schematic views illustrating reduction gear sets and differential gear sets of a conventional 4WD hybrid electric vehicle.
Referring to FIG. 1, a motor generator (MG) 1 is arranged for a rear wheel drive, a reduction gear set 3 and a differential gear set 5 are arranged to transmit driving force of the motor generator 1 to left and right wheel shafts 6a and 6b. 
In a power transmitting structure of FIG. 1, a three-shaft structure is employed which includes a motor output shaft 2 coupled to the motor generator 1, a middle shaft 4 for receiving torque of the motor output shaft 2 through the reduction gear set 3, and the wheel shafts 6a and 6b for receiving driving force of the middle shaft 4 distributed by the differential gear set 5. The motor generator 1, the reduction gear set 3 and the differential gear set 5 cooperate with each other to perform vehicle driving, a regeneration function and a differential function.
As the differential gear set 5, a differential gear set of a straight bevel gear type is used, and three gear sets are used in the power transmitting structure of FIG. 1.
A power transmitting structure of FIG. 2 employs a three-shaft structure which includes the motor output shaft 2, the middle shaft 4 and the wheel shafts 6a and 6b together with the reduction gear set 3 and the differential gear set 5. The power transmitting structure of FIG. 2 further includes an electronic clutch 7. The motor generator 1, the reduction gear set 3 and the differential gear set 5 cooperate with each other to perform vehicle driving and a differential function.
As the differential gear set 5, a differential gear set of a straight bevel gear type is used, and three gear sets are used in the power transmitting structure of FIG. 2.
A power transmitting structure of FIG. 3 employs a single shaft structure which is simpler. A stepped-pinion planetary gear is used in the power transmitting structure of FIG. 3, and the motor generator 1, the reduction gear set 3 and the differential gear set 5 cooperate with each other to perform vehicle driving, a regeneration function and a differential function.
As the differential gear set 5, a differential gear set of a straight bevel gear type is used, and three gear sets are used in the power transmitting structure of FIG. 3.
All of the rear wheel drive apparatuses of the conventional 4WD hybrid electric vehicle described above have the differential gear set of the straight bevel gear type, and employs the three-shaft structure or has three gear sets, whereby the number of parts is high and the structure is very complicated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.